Window glass, or raw soda-lime glass, is naturally relatively highly emissive. To improve thermal efficiency (insulation properties), specially designed thin film coatings are often applied to one or more surfaces of the raw soda-lime glass. These coatings reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated, while letting visible light pass. This results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.
The inward and outward appearance of coated glass can be difficult to anticipate. In addition, the light throughput and the effect thereon can also be difficult to anticipate. Computer modeling programs are often used to attempt to portray the appearance of the coated glass. However, when viewing the portrayal on a computer or printout thereof it can be difficult to obtain an accurate understanding of how the coated glass will appear when actually constructed and in place on a building or other structure.